Circulating progenitor cells (CPCs), including endothelial progenitor cells (EPCs), have a key role in endothelium repair. Cellular NADPH oxidase (Nox) enzymes, including Nox-containing gp91phox, represent a source of reactive oxygen species (ROS); ROS trigger protective signals but may also have detrimental effects. Cellular defenses against ROS include the enzymes manganese superoxide dismutase (MnSOD), catalase (CAT) and glutathione peroxidase type-1 (GPx-1). We investigated the relationships of CPCs with cellular gp91phox, MnSOD, CAT, GPx-1 and ROS levels and with carotid intima-media thickness (cIMT) and stiffness in hypertensives without additional risk factors for cardiovascular disease. CPCs from 53 newly diagnosed, untreated hypertensives and from 29 controls were isolated and identified by flow cytometry. gp91phox, MnSOD, CAT, and GPx-1 mRNA and protein expression and ROS generation were evaluated in enriched samples of CD34(+) cells; cIMT and stiffness were assessed. Hypertensives showed higher arterial stiffness (P < 0.001) but no difference in cIMT with respect to controls. ROS generation was slightly increased (P=0.04), whereas gp91phox, MnSOD, CAT and GPx-1 were significantly higher (P < 0.001) with respect to controls, as was CPC number (P < 0.001), but EPCs were no different. CPC and EPC numbers correlated with gp91phox, ROS and fibrinogen (P < 0.001); moreover, gp91phox, MnSOD, CAT and GPx-1 were correlated with CPC number. In early phases of arterial hypertension, before the development of wall thickening and even in the presence of arterial mechanical impairment, CPC number may be increased to maintain an adequate number of EPCs in peripheral blood.